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General Chemistry Principles, Patterns, and Applications, 2011

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In contrast, recall that half-reactions are written to show the reduction <strong>and</strong> oxidation reactions that<br />

actually occur in the cell, so the overall cell reaction is written as the sumof the two half-reactions.<br />

According to Equation 19.10, when we know the st<strong>and</strong>ard potential for any single half-reaction, we can<br />

obtain the value of the st<strong>and</strong>ard potential of many other half-reactions by measuring the st<strong>and</strong>ard<br />

potential of the corresponding cell.<br />

Note the Pattern<br />

The overall cell reaction is the sum of the two half-reactions, but the cell potential is the difference<br />

between the reduction potentials: E°cell = E°cathode − E°anode.<br />

Although it is impossible to measure the potential of any electrode directly, we can choose a reference<br />

electrode whose potential is defined as 0 V under st<strong>and</strong>ard conditions.<br />

The st<strong>and</strong>ard hydrogen electrode (SHE) is universally used for this purpose <strong>and</strong> is assigned a st<strong>and</strong>ard<br />

potential of 0 V. It consists of a strip of platinum wire in contact with an aqueous solution containing 1 M<br />

H + . The [H + ] in solution is in equilibrium with H2 gas at a pressure of 1 atm at the Pt-solution interface<br />

(Figure 19.6 "The St<strong>and</strong>ard Hydrogen Electrode"). Protons are reduced or hydrogen molecules are<br />

oxidized at the Pt surface according to the following equation:<br />

Equation 19.11<br />

2H + aq<br />

One especially attractive feature of the SHE is that the Pt metal electrode is not consumed during the<br />

reaction.<br />

( ) + 2e- H2( g)<br />

Figure 19.6 The St<strong>and</strong>ard Hydrogen Electrode<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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